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Indicators of Phosphorus Deficiency and Excess

• Deficiency: interference in reproductive processes (delay in flowering), reduced longitudinal growth, dwarfing, smaller leaves because

2.3.5.1 Magnesium

Function

Similar to other alkaline cations, magnesium is not metabolised and it is therefore difficult to determine its actual function.

Sucrose \

Phloem loading

Structural function of Mg:

• in the structure of chlorophyll (central atom of four pyrrole rings)

• in the cross-connections of cellulose fibrils in the cell wall

Metabolic functions of Mg:

• stabilising enzymes, predominantly during the turnover of phosphates (nitrogenase, ATPase, phosphorylase, and others). In these processes Mg binds to phosphate and, because of allosteric interactions, binding between substrate and phosphate becomes possible

• stabilising energy-rich bonds, e.g. phytate as phosphate storage

• Mg regulates the proton gradient in the stroma of chloroplasts in ATP synthesis and thus determines the pH in the chloroplast (pH 7.6 in the dark and 8 with illumination)

• osmoregulation and pH regulation in the cell, as antagonist to Ca and K.

Uptake and Requirement

In the soil magnesium is bound to the substrate predominantly as an exchangeable cation. Mg is a component of primary and secondary minerals (serpentine) from which it is released by weathering. The uptake of magnesium is particularly antagonistically influenced by ammonium. The most important antagonist for Mg is Ca, but also NH4, K, Mn and even H+ influence the uptake of Mg.

In the plant magnesium is transported in the xylem and phloem and the ion is stored in chloroplasts. Remobilisation from ageing leaves occurs.

Deficiency and Excess

With Mg deficiency and excess the following symptoms occur:

• Mg deficiency: yellowing of older leaves, yellow tips and early shedding of needles, chlorosis because of inadequate chlorophyll synthesis, starch accumulation because of the effect on phosphate metabolism, water relations affected because of poor osmoregulation, dwarf growth.

• Mg excess: on Mg-rich substrates (serpentine) because of interactions with other nutrients. Water stress may lead to increased concentrations of Mg.

2.3.5.2 Calcium

Function

Structural function of Ca:

• stabilising cell wall together with Mg Metabolic functions of Ca:

• Ca2+ is toxic and only transported in the cy-tosol as bound proteins

• interaction with phytohormones (IAA) in elongation of cells (calmodulin)

• interactions with ABA

• activator of membrane-bound enzymes (amylase) and ATPases of ion channels

• osmoregulation (shrinking)

• Ca functions in the mitochondria, in contrast to Mg which regulates the proton gradient predominantly in chloroplasts.

Uptake and Requirement

Ca occurs in the soil solution as Ca2+; a distinction must be made for "Ca-rich" and "Ca-poor" soils. Calcium-rich soils are predominantly found on limestone. The pH of these soils with almost unlimited CaC03 is stabilised by the dissolution of CaC03 at about pH 7. In Ca-poor soils, CaC03 is quickly consumed because it dissolves easily and is not stable against weathering, so that the dissolved Ca2+ is only bound to the ion exchangers of the soil after the dissolution of the carbonate. The pH of Ca-poor soils is usually around pH 5, but may sink on acidic rocks (shales containing pyrite) to 3.5. Below a pH of 5.5 the chemistry of the soil is increasingly determined by aluminium. Ca uptake at the root is antagonistically influenced by other cations, particularly Al. If A1 instead of Ca is incorporated into the cell wall, elongation growth is inhibited.

Ca uptake occurs in symport with anions, particularly nitrate, and in acidic soils with sulfate and occurs predominantly at the root tip, where the endodermis is not yet formed, as transport into the xylem via the cell wall is possible there. Ca is transported in the xylem, but is absent from the phloem where, because of the high pH, Ca would react with phosphate forming insoluble apatite. Thus Ca is transported in the phloem only in bound form (Ca protein binding, e.g. calmodulin). Ca is stored in cell walls and vacuoles together with malate, where Ca-oxalate, Ca-sulfate or Ca-carbonate may be formed by precipitation. High Ca concentrations also occur in the endoplasmic reticulum.

Box 2.3.13

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